Load bearing molded thermoplastic articles for structural applications often have ribs and like features to provide strength. For example, a molded pallet will have an egg-crate pattern of ribs. As another example, a molded arch shaped conduit for dispersing and gathering liquids in the earth, described in U.S. Pat. Nos. 4,759,661 and 5,017,041 (having common inventor and assignee herewith) has ribs in addition to corrugations, so that when buried in the earth it will resist the weight of the earth and vehicles. Such articles are typically made by injecting hot polyethylene into a relatively cold die.
An improved method of making large polyethylene objects like the conduit involves injecting nitrogen or another nonreactive gas into the mold before the thermoplastic hardens. Thus, hollow walls are formed in the articles where they would otherwise be thick. See U.S. Pat. Nos. 4,247,515, 4,234,642, 4,136,220, all to Olabisi, and U.S. Pat. No. 4,101,617 to Friedrich. The technique reduces the injection pressure which is required for flowing plastic to the far reaches of the mold, reduces the amount of polymer needed, and speeds the injection molding cycle time. When ribbed products are made, the thicker cross section ribs will desirably be hollowed.
As the foregoing gas assisted injection molding process is commercially known and is related to ordinary injection molding, many essential principles for product shape and mold design are known. However, there are process limitations which give less than the desired product properties and dimensions. One problem is a tendency of ribs to have areas which are not desirably hollowed out, since gas sometimes pushes molten plastic from opposing directions whereupon it is trapped and forms a block, as described in U.S. Pat. No. 4,234,642. Such a region will not cool as fast as the rest of the adjacent hollow structure, undercutting the expected faster cycle time, or producing localized distortion and surface shrinkage. Another problem occurs, as in the leaching conduit, where a strengthening rib butts against an undulation of the corrugation, insofar as controlling hollowness and obtaining good strength at the butt joint. Still another problem results when residual pressurized gas, trapped in the molded part, upon removal causes wall eruptions at locations which are insufficiently cooled or have very thin walls. Thus, there is a need for improvment in the technology of molding and design for this kind of product and process.